Liquid crystal electrooptic devices are well known. The devices comprise a layer of liquid crystal material filling the space formed between two parallel electrodes, e.g., glass plates coated with a thin, conductive film, such as of tin oxide or indium oxide. The liquid crystal material is aligned in a particular direction with respect to the electrode plates. When a voltage is applied to the electrodes, the liquid crystal molecules realign.
Dynamic scattering liquid crystal cells contain liquid crystal compositions having negative dielectric anisotropy. These compounds, originally aligned perpendicularly to the electrodes, become turbulent in an electric field, thereby scattering light.
Field effect liquid crystal cells contain liquid crystal materials having positive dielectric anisotropy. These compounds, originally aligned in a direction parallel to the electrodes, realign in a direction perpendicular to the electrodes in an electric field. This phenomenon is made visible by a polarizer and analyzer external to the cell.
In order to optimize the contrast between the "on" and "off" states, it is desirable to maximize the original alignment of the liquid crystal material. This has been done in several ways; for field effect cells, the electrode plates can be rubbed or scratched in a single direction prior to fabricating the cell, or, preferably they can be given a thin, slant evaporated coating, as of a silicon oxide, which does not interfere with the electrodes but which does form a series of microgrooves along which the liquid crystal molecules sympathetically align.
A homeotropic aligning agent is generally added to dynamic scattering liquid crystal compositions in order to improve the original alignment. This is a chemical agent which improves the wettability between the electrode plates and the liquid crystal composition. Such aligning agents are well known in the art.
After preparing the electrode plates, two of them are cemented together with a seal for each cell, preferably a glass frit seal, leaving a single opening as a fill port. The cell is filled by evacuating the cell and immersing in a reservoir of the desired liquid crystal material. A preferred method of filling dynamic scattering liquid crystal cells containing an aligning agent is disclosed in U.S. Pat. No. 4,064,919 of Stern et al, incorporated herein by reference.
When a dyestuff is added to the liquid crystal material and the liquid crystal cell is vacuum filled, difficulty is encountered in maintaining a homogeneous mixture in the cell. The dyestuff tends to concentrate in areas around the fill port, causing visible inhomogeneities in the color of the cell which detracts objectionably from its appearance. Further, upon standing, a localized high concentration of dye tends to precipitate out of solution. Thus it would be desirable to find a method of improving the homogeneity and stability of liquid crystal -- dyestuff mixtures in liquid crystal cells.